Work on the comparative folate-related metabolism of filariae and mosquito vectors of filariae will continue, with high priority assigned to two aspects. First, preliminary findings indicate that filariae, unlike mosquitoes, lack the capacity for cobalamin-dependent methionine synthesis. This point must be established beyond doubt; whether filariae are capable of cobalamin-independent methionine synthesis will also be determined. The general nature of methionine metabolism in filariae will be studied. Second, the physiological roles of 10-formyltetrahydrofolate synthetase, very active in both mosquitoes and filariae, will be investigated. Filariae, unlike mosquitoes, seem to be incapable of synthesizing purines de novo, so that one important function of the folate cofector formed by this enzyme, 10-formyl FH4, can likely be ruled out in the former.